Genetic recombination is a universal biological process which occurs in almost all organisms. It is a major source of the variation upon which natural selection acts. Our overall objective in this proposal is to understand it in a simple eucaryotic cell, Saccharomyces cerevisiae, at a genetic and molecular level. The formal genetic properties of recombination in S. cerevisiae (baker's yeast) are identical to those found in higher eucaryotes; the ease with which yeast can be manipulated both genetically and biochemically, however, gives a distinct advantage over more complex cells. We propose to understand genetic recombination by: 1) Monitoring the effects of genetically well studied Rec mutants on simple substrates (e.g., plasmids or artificially created intermediates) that can be assayed both genetically and biochemically. This will allow hypotheses for the Rec gene function to be formulated as well as give insight into the mechanism of recombination. 2) Recombination does not occur uniformly along the chromosome; thus, we will characterize a sequence of DNA that exhibits high frequencies of recombination in meiosis. These experiments will help discern the role that DNA plays in initiation (or termination) of recombination events. 3) Isolate and genetically analyze new Rec mutants by taking advantage of the technique of reverting known Rec mutants. Subsequent genetic analysis will include the construction of recombination pathways in meiosis and mitosis by analyzing interactions between the various Rec mutants. The process of genetic recombination clearly is of general importance in higher eucaryotes (e.g. human cells). In somatic cells, it is implicated in carcinogenesis (by virtue of homozygosing recessive markers and in rearranging chromosomes), expression of immunoglobulin genes, and in aging (by virtue of the interactions of repair and recombination functions). In meiotic cells, recombination is required for proper chromosome segregation at meiosis I. Since nondisjunction at meiosis I is a major source of chromosome abnormalities (Trisomy 21, etc.), it is clear that recombination plays a role in the generation of birth defects. We feel that understanding the process in a simple system will give insight into more complex ones.